1. Field of the Invention
The present invention relates to a technique to reproduce data on a recording medium, and more particularly to an offset correction technique for appropriately controlling a low frequency signal added to an input signal.
2. Description of the Background Art
In recent years, a disc apparatus with high density and large capacity such as a magnetic disc apparatus or an optical disc apparatus represented by a DVD (Digital Versatile Disc) has actively been studied and developed. Since such a disc apparatus has achieved higher density, a PRML (Partial Response Maximum Likelihood) detecting scheme has become indispensable as a signal processing technique for improving an error rate of reproduced data. It is well-known that this scheme combining partial response waveform equalization with maximum likelihood detection considers the frequency property from a disc to an input of a maximum likelihood detector as a “coder”, and corrects a reproduction signal with waveform equalization followed by decoding with maximum likelihood detection.
PRML detection is particularly effective in a case in which, when information highly densely recorded on an optical disc is reproduced, intersymbol interference becomes significant to lower reproduction amplitude in high frequency, and the SN ratio of a high-frequency component of the reproduction signal is deteriorated to aggravate the error rate. In the maximum likelihood detection scheme, a pattern minimizing a total sum of a square error with respect to an error between input data and an ideal expected value obtained from a property of the coder is selected from all patterns, with respect to an amplitude information column input to the maximum likelihood detector and having approximately 8-bit quantized bit number, for example. Meanwhile, it is difficult to perform a processing described above in an actual circuit in terms of a circuit size and an operation speed. Therefore, the processing is usually performed with the use of an algorithm called “Viterbi decoding.”
As an example of PRML detection combining partial response waveform equalization with Viterbi decoding as above, Japanese Patent Laying-Open No. 6-243598 proposes PRML detection using PR (1, 2, 1) property with a ratio (1:2:1) of impulse response of an isolated mark assumed by a Viterbi decoder.
On the other hand, data recorded on the optical disc is normally coded in advance by an RLL (Run Length Limited) code as a recording code. A variety of codes have been proposed, and in summary, the feature consists in limiting the number of 0s located between 1s. This limitation is generally called (d, k) limitation and denoted by (d, k) RLL code or the like. The (d, k) limitation means that the number of 0s located between 1s never fails to be not smaller than d and not larger than k. Normally, after the data is modulated to the RLL code, it is further subjected to NRZI conversion. The NRZI conversion is a scheme to invert 1 every time 1 is detected in the data prior to NRZI conversion. By combining NRZI conversion with (d, k) RLL, a minimum inversion interval is set to (d+1), while a maximum inversion interval is set to (k+1). In an example of (1, 7) RLL code where d=1 and k=7, the number of 0s located between 1s is not smaller than 1 and not larger than 7. Therefore, after NRZI conversion, the minimum inversion interval is set to 2, and the maximum inversion interval is set to 8. A detection scheme combining limitation of the minimum inversion interval with PRML detection is extremely effective as a scheme attaining further improvement in a decode error rate. Japanese Patent Laying-Open No. 7-122000 shows a detection scheme combining a property of the minimum inversion interval not smaller than 2 with PRML detection using PR (1, 2, 1).
Though Viterbi decoding (maximum likelihood decoding) above is highly effective means to enable recovering of SN deterioration, it is effective only for an example in which white noise is superposed on the reproduction signal. Performance is considerably deteriorated when direct-current level fluctuation, that is, a low-frequency noise is caused, as in a normal binary detection scheme.
The direct-current level fluctuation, that is, the low-frequency noise is attributed, for example, to a disc substrate or a medium, or to removal of a low-frequency component in the recording code such as (d, k) RLL code+NRZI conversion by a high pass filter within a signal reproduction system. In addition, occurrence of duty deviation of a mark on a disc, namely what is called “asymmetry”, also causes the direct-current level fluctuation.
Several techniques to avoid deterioration of Viterbi decoding performance due to the direct-current level fluctuation have conventionally been proposed. For example, a scheme shown in Japanese Patent Laying-Open No. 6-325504, aiming to prevent deterioration of the performance of the Viterbi decoder due to the direct-current level fluctuation, detects a transition pattern that a level of the reproduction signal varies across a center level, and uses amplitude data in that pattern to correct direct-current offset.
In addition, in a scheme shown in Japanese Patent Laying-Open No. 10-172238, an error between an input sample value and a reference level is detected, and information from a Viterbi detector is used to extract the error corresponding to a survivor path (selected path) of Viterbi decoding. Then, an average of n errors is employed as a direct-current offset correction value.
If the duty deviation of the mark on the disc, namely what is called “asymmetry”, is causing the direct-current level fluctuation, apparently, a difference between a proportion of a direct-current component superposed on a high-frequency signal and a proportion of a direct-current component superposed on a low-frequency signal is created. Moreover, as a signal has a higher frequency, the direct-current level tends to be shifted. Accordingly, an eye pattern exhibits a waveform in which the center of the eye is deviated from the amplitude center, as shown in FIG. 7. In an example in which the white noise is superposed on such a reproduction signal, when the reproduction signal is quantized at a timing indicated with an arrow in FIG. 7, it is observed in an amplitude histogram that each crest of the histogram apparently has a separate offset, as can be seen in FIG. 8.
When an attempt is made for offset correction of such a signal, the schemes disclosed in Japanese Patent Laying-Open No. 6-325504 and Japanese Patent Laying-Open No. 10-172238 do not consider the direct-current offset deviation due to asymmetry. Therefore, optimal direct-current offset adjustment has not been attained.